1. Field of the Invention
The present invention relates to inkjet printers adapted to eject ink drops onto a sheet for an image to be formed thereon while the sheet is being transported.
2. Description of the Related Art
FIG. 3 is a perspective view of a conventional inkjet printer, and FIG. 4 is a schematic side view of the same. As shown in FIGS. 3 and 4, a sheet is picked up and fed by a not shown pick-up roller, nipped between a transport roller 4′ and a plurality of driven rollers A′, and transported a predetermined distance leftward in FIG. 4, i.e., in a slow scanning direction. An inkjet print head 22 is held by a carriage positioned downstream of the roller 4′ in the slow scanning direction. While being reciprocated in a fast scanning direction and an opposite fast scanning direction, the head 22 ejects ink drops onto a sheet. The fast and opposite fast scanning directions are perpendicular to the slow scanning direction. The printer performs an image forming process in a sequence of alternating transport of sheet in the slow scanning direction and reciprocation of the head 22 in the fast and opposite fast scanning directions. The printer stops the reciprocation of the head 22 when a not shown sheet sensor detects a tailing end of a sheet being about to pass through a nip area between the roller 4′ and the rollers A′. The sheet is finally nipped between output rollers 5′ and output driven rollers 7′, to be ejected from the printer. The foregoing arrangement has the following problem. A distance X between the position of nozzles of the head 22 and the location of the nip area causes a blank space in a tailing end of sheet where no image is formed. In other words, the tailing end has a limited image area.
Some conventional inkjet printers allow an image to be formed up to a tailing edge of sheet without leaving a blank space in such mechanical methods as of: transporting a sheet at a low speed during duration of image formation on a tailing end of the sheet; increasing nip pressure applied to a sheet by output rollers in order to reduce a change in load on a sheet caused at the moment a tailing end of the sheet is released from transport rollers; providing a mechanism for reducing backlash in a drive system for transport rollers with springs; or improving dimensional accuracy of components such as rollers. Also provided with complicated inkjet control system designed for high-quality image formation, the conventional printers have problems of complicated control of image formation and increased component costs. With regard to a leading end of the sheet, the sheet undergoes a change in load thereon the moment the leading end is nipped between the rollers 5′ and 7′, as with the tailing end. The change in load leads to improper transport of the sheet, thereby causing image deterioration such as color unevenness. Conventionally, the output driven rollers 7′ are generally aligned with one another along the fast scanning direction. The moment a leading end of sheet is nipped between the rollers 7′ and the roller 5′, thus, the rollers 7′ simultaneously apply pressure to the leading end, thereby causing a negative effect on sheet transport precision. The sheet transport precision decreases at higher pressures of the rollers 7′.
FIGS. 5 and 6 are views illustrating configuration of another conventional inkjet printer provided as a solution to the problems. The printer has driven rollers A′, B′, and C′ arranged at three different locations along the slow scanning direction. Thus, there are three locations of nip areas between a transport roller 14 and the rollers A′, B′, and C′. The single roller C′ is arranged in the center along the fast scanning direction. The two rollers A′ are positioned symmetrically with respect to the roller C′, and so are the two rollers B′. Five holders 18 are provided for holding the respective rollers A′, B′, and C′ rotatably. The holders 18, which are respectively paired with the rollers A′, B′, and C′, have respective springs 19 for urging the rollers A′, B′, and C′ against the roller 14.
The arrangement allows pressures applied to a sheet by the rollers A′ to C′ to be decreased, from upstream to downstream, sequentially in three steps when a tailing end of the sheet is released from the three nip areas. Thus, the arrangement results in a reduced change in load on the sheet the moment the sheet is released from pressures between the roller 14 and the rollers A′, B′, and C′. The reduced change in load means less negative effect on sheet transport precision and better image quality. The printer is otherwise similar in component and operation to the first conventional printer as shown in FIGS. 3 and 4.
In order to prevent improper image formation in a tailing end of sheet, there have also been proposed inkjet printers provided with driven rollers of flexible and hard materials aligned in two lines along the slow scanning direction, or provided with an auxiliary member, of various materials or shapes, that is located downstream of driven rollers and adapted to apply pressure to a sheet (see Patent Literature 1). Further, there have been proposed inkjet printers that are adapted to control pressures applied by driven rollers to a sheet so that the pressures can be almost or entirely eliminated (see Patent Literature 2 and 3). p [Patent Literature 1] JP H05-186086A p [Patent Literature 2] JP H07-033279A p [Patent Literature 3] JP H11-208923A